Yup. I've been a server before myself, and delivered pizzas, and unless the service is abyssmal, my minimum is 20%. (By abysmal, I mean I get seated and forgotten about for 15-20 minutes before anyone takes my drink order; then another 15 goes by before my order gets taken; my drink is more often than not empty; the server has an attitude, etc.)
If gratuity is added to a check, and the service was good, I'll even leave a little more for the server because they deserve more than the standard 17-18%. But, I'm also the kind of person that stacks my own dirty dishes, and wipes up my own messes, because I just feel bad for asking others to clean up after me.
But, if gratuity is added first, and the service was terrible and it's obviously the server's fault, then yeah, I'm calling a manager over and complaining. I'll either get free stuff, or they'll take the gratuity off and I'll leave what I think the server deserves. I
never leave no tip.